Chinese police officers march across Tiananmen Square outside the Great Hall of the People during a hazy day in Beijing, China, Sunday, March 9, 2014. China will tighten environmental legislation and force polluters to pay compensation following renewed blasts of toxic air, the country's top legislator said Sunday. (AP Photo/Ng Han Guan)

Two children have died and another 30 are sick in a mass poisoning at a kindergarten in southwestern China, authorities have confirmed.

Investigators identified the toxic substance as a powerful rat poison, but do not yet know how it was administered Wednesday in the kindergarten in Yunnan province's rural Qiubei county.

Along with the victims, aged 4 and 5, 12 others remained hospitalised Friday, three of them in serious condition.

Chinese schools have suffered a series of mass stabbings and other attacks by mentally disturbed people or those bearing grudges. There have also been incidents of school staff administering medication without authorisation.